1. Field of the Invention
The present invention relates to an apparatus for introducing a pressurized gas and an injection molding method for producing a molded article having a hollow portion with the apparatus for introducing a pressurized gas.
2. Description of Related Art
In a conventional method of introducing a pressurized gas into a molten resin injected into a cavity provided in a mold on the basis of an injection molding method to produce a molded article having a hollow portion (to be sometimes referred to as “gas injection method” hereinafter), the following two methods are known for introducing the pressurized gas into the molten resin injected into the cavity.
In a first method, a gas amount (pressure and volume) required for one operation of injection molding is measured in a cylinder in advance with a piston-type compressor having a piston and the cylinder having a predetermined volume. In the injection molding, the gas measured and accumulated in the cylinder in advance is introduced into a molten resin in a cavity while compressing the gas by means of movement of the piston in the cylinder, to form a hollow portion inside a molded article. Then, the position of the piston is maintained in that state, to maintain the gas pressure within the hollow portion. This method will be called “measuring method” hereinafter. This measuring method is disclosed, for example, in JP-A-60-24913.
In a second method, a gas in a gas source is pressure-increased to a high pressure with a compressor in advance and reserved in a pressure container having a large capacity, and, for injection molding, the gas in the pressure container is introduced into a molten resin in a cavity through a discharge valve or a pressure-regulating valve disposed on the discharge side of the pressure container. This method will be called “constant pressure method” hereinafter. The constant pressure method is disclosed, for example, in JP-A-1-128814.
However, the above prior techniques have the following problems.
(1) Problems of the Measuring Method
(1-1) The cylinder has a constant volume. When a hollow portion in a molded article has a large volume, it is sometimes impossible to generate a gas pressure required for forming the desired hollow portion even if a maximum gas amount is measured. In this case, no molded article having a good product quality can be obtained.
(1-2) When the amount of a gas to be introduced into a molten resin is large, it is required to increase the pressure of a gas to be supplied to the cylinder at a measurement time. In many cases, however, a gas cylinder is used as a gas source, so that the consumption efficiency of the gas cylinder is degraded. That is, when the pressure in the gas cylinder decreases, the gas cylinder can be no longer used.
(1-3) When a hollow portion to be formed in a molded article is large, it is required to increase the volume of the cylinder so that the amount of a gas to be introduced into a molten resin may be increased. It is therefore sometimes required to provide gas compression equipments having different capacities depending upon the volumes of the hollow portions.
(1-4) When the volume of a hollow portion to be formed in a molded article is small, part of a measured gas is of no use, which results in poor economic efficiency.
(1-5) The gas is pressure-increased at one compression step, so that the compression ratio is large. It is therefore required to design the piston diameter, the internal diameter of the cylinder and the piston stroke to such sizes necessary for a one-time full operation of injection molding. As a result, the gas compression equipment is caused to increase in size, the sealing structure for a high-pressure gas is complicated, and the cost for manufacturing the gas compression equipment increases. In addition, the energy consumed for operating the piston is large.
(2) Problems of the Constant Pressure Method
(2-1) The gas pressure in the pressure container is required to be higher than the pressure of a gas to be loaded into a molten resin in a cavity, so that the energy efficiency is poor.
(2-2) The gas pressure in the pressure container is constantly maintained at a high pressure, so that the pressure container involves danger of explosion.
(2-3) When the gas leaks out of a formed hollow portion after the gas is introduced into a molten resin in a cavity, a large amount of the gas from the pressure container hangs around the mold, so that workers may be endangered. In the gas injection method, further, nitrogen gas is generally used as a gas. When the nitrogen gas concentration in a working atmosphere sharply increases, therefore, lack of oxygen may be caused on workers.
(2-4) It is difficult to stably introduce a constant amount of a gas into a molten resin in a cavity in many cases.
It is therefore an object of the present invention to provide an apparatus for introducing a pressurized gas, which enables reliable introduction of a pressurized gas having a predetermined volume required for forming a hollow portion when a molded article having the hollow portion is produced by injecting a molten resin into a cavity provided in a mold and introducing the pressurized gas into the molten mold in the cavity and which has a simple structure, and an injection molding method for producing a molded article having a hollow portion with the above apparatus for introducing a pressurized gas.